How wonderful it all was, he thought--the up and down stroke of the piston in and out of the cylinder, which oscillated from side to side guided by the eccentric; with the steady systematic revolution of the shaft, borne round by the crank attached to the piston-head, all working so smoothly, and yet with such resistless force! The whole was a marvel to him, as indeed it is to many of us to whom a marine engine is no novelty. "Well, my young philosopher," said the Captain, tapping him on the shoulder and making him take off his gaze for a moment from the sight, "do you think you understand the engines by this time, eh?" Bob only needed the hint to speak; and out he came with a whole volley of questions. "What is that thing there?" he asked, "the thing that goes round, I mean." "The paddle-shaft," replied the Captain; "it turns the wheels." "And that other thing that goes up and down?" "The piston-rod," said the old sailor. "It is this which turns the shaft." "Then, I want to know how the piston makes the shaft turn round, when it only goes up and down itself?" "The `eccentric' manages to do that, although it was a puzzle for a long time to engineers to solve the problem--not until, I believe, Fulton thought of this plan," said the Captain; and, he then went on to explain how, in the old beam-engine of Watt, as well as in the earlier contrivances for utilising steam-power, a fly-wheel was the means adopted for changing the perpendicular action of the piston into a circular motion. "Of course, though," he added, "this fly-wheel was only available in stationary engines for pumping and so on; but, when the principle of the eccentric was discovered later in the day, the previously uneducated young giant, `Steam,' was then broken to harness, so to speak, being thenceforth made serviceable for dragging railway- carriages on our iron roads, and propelling ships without the aid of sails, and against the wind even, if need be!" "But what is steam?" was Bob's next query. "That's what I want to know." This fairly bothered the Captain. "Steam?" he repeated, "steam, eh? humph! steam is, well let me see, steam is--steam!" Bob exploded at this, his merriment being shared by Nellie and Mrs Gilmour, the latter not sorry for the old sailor's "putting his foot in it" by a very similar blunder to that for which he had laughed at her shortly before; while, as for Dick, the struggles he made to hide the broad grin which would show on his face were quite comical and even painful to witness. The Captain pretended to get into a great rage; although his twinkling eyes and suppressed chuckle testified that it was only pretence all the time, though his passion was well simulated.